The present invention relates to wrapping machinery for wrapping, or for assisting an attendant to wrap, articles in heat sealable polyvinylchloride (PVC), or other plastic film. Such wrapping machinery is used primarily in supermarkets to wrap food products such as meats, vegetables, fruits, and the like prior to placing such items in the display counters. Usually the food products are placed in a plastic or cardboard tray before being wrapped in the plastic film.
In wrapping apparatus of the type being discussed, a web of continuous plastic film is drawn by an attendant from a supply roll mounted on the wrapper machine or console and the film is wrapped by the attendant in tubular fashion around the food product including the tray, if a tray is used. The attendant then severs the film from the supply roll by using a thermal cut-off device. After completing the wrapping, the package is placed on a pedestal hot plate to seal the film and the package.
In former wrapping mechanisms, the thermal cut-off device was a hot wire of small diameter, for example No. 23 gauge Nicrome wire having a diameter of 0.0226 inch. Because of the small cross-sectional area of such a hot wire, the quantity of heat stored therein was small, and the temperature dropped rapidly upon contact with the film. As a consequence, unless the initial temperature was very high, insufficient heat was available to sever the film. It was found, for example, that it was necessary to operate the small hot wire at a temperature of the order of 700.degree. F. in order to effect satisfactory severence of the film. While this high temperature was sufficient to completely sever the film, it had the undesirable effect of overheating the film and causing smoke to be generated and emitted into the room in which the wrapping was being performed.
To overcome the generation of smoke, the prior art went to the use of a tubular rod of sufficient diameter and cross-sectional area to provide sufficient mass to store a guantity of heat so that the heated tubular rod was able to function as a reservoir of heat to replace that lost at the film contact line. In lieu of a tubular rod of sufficient cross-sectional area, the prior art also, for the same reason, used a rectangular blade of sufficient mass to store a quantity of heat. The prior art found that if the tubular rod or rectangular blade had sufficient mass to store a sufficient quantity of heat, the film could be separated and severed at the relatively low temperature of 325.degree.-350.degree. F. This temperature was low enough to avoid generation of objectionable smoke, and because the tubular rod or blade had sufficient mass to function as a reservoir or supply of instant heat, the line of contact with the film was maintained at a substantially constant and adequate temperature throughout the cutting period. The quantity of heat supplied and available was, however, not sufficient to completely sever the film but was sufficient to soften and score the film along the contact line to permit severence by having the attendant pull the film under tension. This technique has been used for some time and has been found to be satisfactory.
In the prior art wrapping machinery, using the technique just described, the food product and tray, after being wrapped in the plastic film, is placed by the attendant on a pedestal hot plate to heat seal the wrapped film and the package. The temperature used at the hot plate is higher than that of the thermal cut-off device which is used to soften and score the film to permit severence of the film by pulling. Since two different temperatures are involved, it has been customary in the prior art to use separate sources for supplying electrical energy to the hot plate on the one hand and to the thermal cut-off device on the other. Specifically, a first heat source, a first control sensor, and a first control circuit are used to supply heat to the hot plate, and a separate second heat source, a second control sensor, and a second control circuit are used to supply heat to the thermal cut-off.